Will Split-Cycle Engine Compete With EV Powertrains?
One cylinder of Scuderi's split-cycle engine performs intake and compression, while the other handles power and exhaust. The engine completes all four strokes in one crankshaft revolution. (Figure courtesy of Scuderi Group.)
1. Forget comparing combustionable pressures to the lower precompression pressures of a supercharger. It is not a two stroke - it is a split cycle four stroke. There are some distinct advantages wrt to cylinder sealing in a noncorrosive, non-explosion, lower temperature environment.
2. I do not see any extra piston friction - it could be lower. Two complete cycles occur in 720 degrees - current engines [to my knowledge cannot do this]. In 'computer talk', I would describe the engine as a four cycle engine with 2 cycles of pipeline.
3. It would appear to me that allowing the cylinder to be designed for a more focused task - power or compression - would eliminate compromises that are required in todays engines.
4. I see advantages to firing the power cylinder just after TDC *IF* the mixture can be ignited quick enough. I noticed 2 spark plugs and no mention of the fuel injection system.
5. Todays engines "are not my father's engines" - they are more complex. It is not clear that significant additional complexity would be required - beyond the 'magic retention tank'. I am a bit skeptical - but I do not have enough info to comment.
6. The text "When tested on several European economy-class vehicles (including an Audi A1 and a Citroen C1), the engine boosted fuel efficiency from an average of 53.5mpg to 65.4mpg, Scuderi said. It also lowered CO2 emissions from 104g/km to 85g/km." leads me to believe that it was tested in an actual vehicle. The fact that it appeared right after SWR simulations makes me wonder.
7. The *idea* of separating the cycles sounds very appealling temperature, different bore/stroke combos BUT the proof is in the pudding.
I have been dealing with engine efficiency for over ten years with the product of the
Sonic Spark Plug. In todays market, we have a complexity of products that claim increases of fuel mileage. The market customers are squeezed with increase of the cost fuel, and the cost of new vehicles with their complexity of design costs.
To increase fuel mileage, the industry has gone to turbo chargers that work. However, there is a down side to this path. The air charge is 78% Nitrogen. Turbo Chargers, the lean burn techniques, Hydrogen inputs increase the combustion energy with the splitting of the Nitrogen Molecule that releases 236,000 of calories per mole. Then, there is the increase of Nitrogen Oxides, NOX, with its environmental effects. And, the Hydrogen Inputs coupled with Nitrogen Oxides will form Nitric Acid. The biological effects of the NOX components are not fully recognized other than biological with asthmatics. However, the effects on the Photosynthesis System should be studied.
The Sonic Spark Plug is the simplest. and least expensive product that improves engine efficiency by the shattering of fuel droplets, and homogenizing the air/fuel mixture. With a tightening of market price, the addition of Turbo Chargers added expense, and the costs of expensive systems to increase mileage, and power may eventually be out priced in the market.
Josborne & JesseL, I concur completely, this is an (interesting) variation of a miller cycle engine, perhaps even closer to an Adkinson cycle engine, the difference being that instead of an external supercharger, there are TWO sources of pressure induction...a turbocharger AND what is essentially a direct-drive internal supercharger. Could this be a good design...perhaps, but until there are working engines IN REAL CARS, ON REAL ROADS, the issue of drivability, efficiency, and NVH are unknown. What makes me most suspect is the designers claims that this is a game-changing new design. Show me the beef....
This seems only possible in a stationary industrial application. The ignition atdc pretty much rules out rapid speed changes and taking advantage of flow dynamics in the power cylinder. I sense a money grab based on hype.
Your observation about the abundance of screw type compressors is correct, but efficiency is not the only aspect to consider. The actual popularity of the Screw type compressors that have almost replaced the older piston types is more related to other important aspects: smaller size, lower weight, relative freedom from vibration, relative freedom from pulsations, easy to be directly coupled to common speeds of electric motors without speed reducing gearboxes or belts and pulleys; additionally, the modern capabilities to manufacture the rotors with computerized machinning explains their cost becoming lower compared to the situation 15-20 years ago. As you say, most common AIR compressors are now screw types, being that for most common air pressure requirements, they can produce it in a single stage easily. For higher pressures, they are not that advantageous, and at very high pressures piston types are more adequate for the task. Compared to dynamic types, thermodynamic efficiency in piston types is higher due to the process of the compression cycle is more towards isothermic than the polytropic cycle more closely followed in dynamic types; in screw compressors cooling-sealing oil is sprayed into the casing to aid sealing and actually remove a good part of the gas heating during compression, but that requires oil injection (and posterior removal). In dry type screw compressors, there are some "Slip" looses accounting for a lower overall efficiency.
As I'm not close to office at this moment, I can't give you specific values for the sizes similar to the requirements of the supercharger in an Automotive IC engine, but I plan to write them soon.
I'm skeptical on this concept. How is efficiency improved when an extra non-power producing cylinder with its parasitic friction is part of the package?
In high-rpm and high-performance engines, ignition before TDC is necessary because of the time it takes to establish combustion. Therefore a slow turning engine would likely be less efficient with a lot of advance ignition timing because early combustion will produce pressure against the rising pistion. Ignition after TDC as a means to improve efficiency/performance is a necessity in the split-cycle engine, but is illusory in a high speed engine.
Scuderi's concept is not new as earliest concepts of supercharging included an extra cylinder. Even Scuderi admits the addition of the turbocharger made the difference, and I think that's the real efficiency enhancer. I sense smoke and mirrors here, and see academics on a grant-hunting expedition.
I'll be glad to revise my opinion when we are past the paper studies and see some actual dynamometer comparisons between conventional, turbocharged, and Scuderi engines.
Retarding timing reduces negative work, but it also reduces peak pressures (which negatively affects burn efficiency) and combustion time (which negatively affects burn efficiency). When ignition happens at or after top dead center, the flame front has to chase a retreating piston and the flame front move more slowly because of the falling combustion chamber pressure & temperature.
Their unconventional ignition timing implies to me that they are running into detonation issues; probably due to too much boost, poor scavenging, excessive heat, and a poorly shaped combustion chamber. They might make up some of the loss in burn efficiency by using multiple spark plugs, but it strikes me as a kludge.
Ivan: Here's Scuderi's answer to the question that you and many other readers posed: "Firing after top dead center (when the piston is on the way down) puts more energy on the piston's downward stroke which results in more power on the crankshaft. In a conventional engine, the spark occurs when the piston is on the way up (and almost at the top), which creates more negative work because the piston is initially fighting against the force before moving down."
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